Leaching reaction zone

Although there is plenty of experimental evidence that ultrasound improves leaching the exact mechanism is not fully understood. Swamy and Narayana [60] have suggested models for leaching in the presence and absence of ultrasound (Fig. 4.4). Normal leaching takes place as the solvent front moves inward and a steady state diffusion occurs through the depleted outer region and is equal to the rate of reaction within the reaction zone itself (Fig. 4.4a). 

Madsen et al. epplied the reaction zone model to the leaching of Butte ore for particles up to 6 in. in diameter. Columns 5 Ft in diameter, containing 5 tons of ore. were used. Tha principal copper mineral was CufS. Equation (9.4-13) in integrated form was used ... 

The majority of trichloroethylene present on soil surfaces will volatilize to the atmosphere or leach into the subsurface. Once trichloroethylene leaches into the soil, it appears not to become chemically transformed or undergo covalent bonding with soil components. When trichloroethylene was absorbed onto kaolinite and bentonite, the nuclear magnetic resonance (NMR) spectra showed no evidence of chemical reactions (Jurkiewicz and Maciel 1995). Because trichloroethylene is a dense nonaqueous phase liquid, it can move through the imsaturated zone into the saturated zone where it can displace soil pore water (Wershaw et al. 1994). 

Class 5 (the least sensitive) soil include Kashtanozem, Brown soil and Sierozem soil zones in the Plateau of Inner Mongolia and the Loess Plateau, Desert soil zones in He-xi-zou-lang and the Talimu River Basin, Subalpine Steppe soil, Alpine Steppe soil and Alpine Desert soil in the Plateau of Tibet. These kinds of soils, belonging to the soil class of Xerosol or Alpine soil, consist of easy weathering minerals such as carbonate. They show alkaline reactions, with weak leaching and sparse vegetation. Those kinds of soils are insensitive to acid deposition. 

Flutson, J.L. and Wagner, R.J. (1992) Leaching Estimation and Chemistry Model. A Process Based Model of Water and Solute Movement, transformation, Plant Uptake and Chemical Reactions in the Unsaturated Zone. Version 3. Dept, of Soil, Crop and Atmospheric Sciences, Series No. 92-3, Cornell University, Ithica, New York. 

Although Eh (redox potential) can affect both the adsorption/desorption reactions that can occur as well as the secondary phases that can form during CCB weathering, Theis Richter (1979) stated that their studies of ash disposal ponds showed that oxidizing conditions rather consistently prevail in the active leaching zone (i.e., the unsaturated zone above the water table). 

Thus for each zone, during a given cycle, the adsorption-desorption process is separated into two distinct events with F or G describing the kinetics of each event. Such an approach is of course valid only for first order rate reactions. In the limit of low concentration, (such as that resulting from slow leaching from a repository) the reaction sites on the rock will not approach saturation and the number of reaction sites can be considered to remain constant during adsorption. Therefore, for a single species in solution at tracer concentrations the reaction should approximate a first order reaction (i.e., where no complications such as concentration effects, step-wise dehydration, dissociation, etc., are present). 

Wagenet, R.J. and J.L. Hutson (1989). LEACHM Leaching estimation model - a process based model for water and soute movement, transformation, plant uptake and chemical reactions in the unsaturated zone. Continuum Vol. 2. Water Resources Institute, Cornell University Ithaca, NY. 

These tests indicated that sulfur-infiltrated concrete still loses abundant sulfur when immersed in neutral and sulfatic solutions, even though the reaction is localized and relatively slow. Compression tests of a few cylinders showed there had been little reduction in strength over several months. Examination of sawn surfaces revealed no clearly leached zone, but the specimens had turned a mottled blue, except near the center. Faint peripheral fractures had developed, and coherence of the infiltrated matrix near the surface had decreased, suggesting that some leaching had taken place. 

Hutson JL, Wagenet RJ. LEACHM Leaching estimation and chemistry model. A process-based model of water and solute movement, transformations, plant uptake and chemical reactions in the unsaturated zone. Version 3. Ithaca, NY Department of Soil, Crop and Atmospheric Sciences. Research series no. 92-3, Cornell University, 1992. 

In humid, tropical climates, weathering is rapid, partly because the high temperatures speed up reactions, but mainly because the consistent supply of heavy rainfall allows rapid flushing and removal of all but the most insoluble compounds, for example oxides of aluminium and iron (Section 4.7). Flushing constantly removes (leaches) soluble components and is particularly important in the undersaturated zone of soils (Box 4.9). 

The residual effects of Mo fertilizers are related to the reactions of the applied Mo with the soil. Mo removal by cropping and by grazing animals, and the extent of Mo leaching from the root zone. One of the earliest reports of marked residual effects from applied Mo was by Anderson (1956). Growth effects due to Mo application at 70gha to subterranean clover Trifolum subterraneum L.) pastures were still evident after 10 years. 

The plant availabiUty of Mo applied to soil will decline with time because of reactions between Mo and soil constituents, leaching of Mo to regions below the rooting zone of plants, and removal of Mo taken up by plants. The relative importance of these processes will depend on the pH of the soil, the sesquioxide content of the soil, and the texture of the soil, as well as on the amount of rainfall. Jones and BelUng (1967) found that 60-95% of added Mo was leached from 16-cm columns of calcareous... 

---